People are talking about conditions on the ground, but one doesn't (normally) fly on the ground.
This plane had a ceiling of 15k-17k feet (dep. on exact type of Lance) and that was plenty enough to fly VMC on top on the day, avoiding any vertical development while remaining VMC.
Presumably ATC will know if he actually did that but somehow got into IMC and iced up. But to fly low down in those conditions doesn't make any sense - unless he didn't have an IR. And flying to the USA without an IR is problematic because much of the flight across the water has to be at a low level too.